Abstract: Fluidized bed gasification technology has wide applications on coal industry due to its excellent mixing and heat transfer performances. With the influence of strong gas-solid mixing, char content of discharged ash will increase at high temperature. The operating temperature of traditional fluidized bed gasification is low to decrease the carbon loss, but it will lead to a shortage of gasification efficiency. As a result, the ash agglomerating fluidized bed gasification allowing limited particle agglomeration is proposed to increase operating temperature. The agglomeration size is the core of ash agglomerating gasifier so it is important to investigate the process of ash agglomeration. Expensive costs will be required if the research of complicated particle behavior in the gasifier depends on experimental analysis only. Thus, it is necessary to combine reactor modeling with experiment to improve research efficiency. However, the actual reaction and transport process of ash agglomerating gasifier cannot be described by existing models because the particle agglomeration was neglected and particle size was average during the modeling process. In this paper, a novel model for ash agglomerating fluidized bed gasifier was proposed: (1) the gasifier is divided into bubbling region consisting of bubble and emulsion phases, and freeboard regarded as pseudo-homogeneous phase; (2) the particle agglomeration is described by the integration agglomeration mechanism considering the particle agglomerating phenomenon, and the initial size distribution of particle. The model was validated according to the comparison that the deviation of simulation results and practical data is within 10%. Finally, the agglomeration size and carbon conversion with different oxygen-coal ratio was calculated by the model. With the agglomeration size reaching the critical value, the maximum carbon conversion is observed. The fluidization of agglomeration cannot keep if the agglomeration size exceeds the critical value.